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Bryophyte Flora of North America

Key to the Moss Genera of North America North of Mexico

Dale H. Vitt and William R. Buck

Based on a key originally published in Contributions from the University of Michigan Herbarium 18: 43-71. 1992, which holds the copyright.

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Traditionally, mosses have been considered by many to present severe difficulties in identification of both species and genera. However, often if the genus is known, species identification follows with much less difficulty. Moreover, most regional moss floras present keys to species beginning at either the family or genus rank. We present here a key to the genera of mosses found in North America, north of Mexico. The key is strictly dichotomous and attempts were made to use gametophytic features whenever possible.

Mosses (Class Bryopsida or Musci) are the largest group of plants in the Bryophyta, and are the third largest class of green land plants in North America following the Monocots and Dicots. The recently published lists of North American mosses (Anderson et al. 1990 [Andreaeidae and Bryidae], Anderson 1990 [Sphagnidae]) tabulate 1320 species in 312 genera. To these, we add the genus Takakia with two species (Murray 1988, Smith et al. 1990). Within the 313 genera of mosses found in North America, 15 have 15 or more species. Only two genera have more than 50 species - Sphagnum with 72 and Bryum with 66; five genera have 30 or species (Orthotrichum - 37, Fissidens - 36, Grimmia and Brachythecium each with 34, and Pohlia - 30). Other large genera are Tortula (29), Dicranum (27), Racomitrium (24), Hypnum (21), Campylopus (18), Hygrohypnum (18), Encalypta (16), and Polytrichum (15). These 15 genera contain 476 or 36% of the North American species.

Historically, keys to mosses have used growth form (acrocarpy vs. pleurocarpy) as a major dichotomy. We have resisted the use of this character whenever possible, and instead placed more emphasis on microscopic features of the leaves; namely the presence, absence and number of costae found on leaves and on the ornamentation of the leaf cells (papillose vs. smooth).

Generic concepts depend largely on previous monographic work completed in particular families. Some North American families have no recent generic revision, while others have newly revised generic concepts in place. We have followed the generic concepts presented in Anderson et al. (1990), including the recognition of segregate genera in the Mniaceae, Amblystegiaceae, and Grimmiaceae. These generic concepts differ somewhat from those accepted in the Canadian (Ireland et al. 1987) and European checklists (Corley et al. 1981).

Floristic treatments of North American mosses are available for many areas of the continent, and date back to the 1880's when Lesquereux and James (1884) published the first treatment of mosses of the continent. Still a valuable source of information and the only flora that treats the southwestern United States is Grout's Moss Flora of North America (Grout 1931-40). In more recent times (since 1970) several excellent regional floras have been written and form the basis of our understanding of North American mosses. Included are the following manuals: Eastern North America - Crum and Anderson (1981); the United States Pacific Northwest - Lawton (1971); Utah - Flowers (1973); northern Michigan - Crum (1973); Maritime Provinces of Canada - Ireland (1982); the Gulf South - Reese (1984); the Interior Highlands - Redfearn (1983); and northwestern North America - Vitt et al. (1988). Definition of terms used in this key follow those given in Crum and Anderson (1981) and in Magill (1990).

We have the great privilege to dedicate this key to Howard Crum, our teacher and good friend. His patience and perseverence in performing the unenviable task of teaching both of us the fundamentals of science and bryology are remarkable. Without him, neither of us would have been able to complete the present work. To Howard Crum we give our heartfelt thanks.


We wish to thank the Natural Sciences and Engineering Research Council of Canada for support of this research, both indirectly through the support of previous field work and laboratory research of DHV and directly by monies that allowed us to work together in Edmonton for a period of time. We would also like to thank Sandra Vitt who provided significant help in organizing our primitive attempts in the use of computers. Willam D. Reese carefully read a draft of this key and made numerous comments for which we are grateful.

Note, Oct. 7, 2001: This key is published here exactly as originally published on the original BFNA Web site at the New York Botanical Garden. It will be revised to reflect new taxonomic changes incorporated into the BFNA. -- R. Zander